The Masked Killer shows up in Lakeview Cabin III as a child, on the Family Portrait, if you manage to get the Family Portrait Ending. In this, he is seen wearing a yellow and green striped shirt.
The Masked Killer is the main, and only, enemy in Lakeview Cabin V. The ultimate goal of Lakeview Cabin V is to kill him.
In game he can appear at any time in any room. He wields a Kitchen Knife, and upon taking any damage, will vanish in a puff of smoke. He cannot appear in Limbo.
After taking damage from the second entity, he will have a wounded face.
After taking damage from the third entity, he can be found, as a child, near The Graveyard. He will have a Knife next to him. Throwing The Box with the spirit of 'E' at him will result in Ending A. Killing him with the Kitchen Knife after bringing him the Football, the Tricycle, and the Toy Ray-Gun will result in Ending B. Killing him otherwise will result in Ending C.